Lags and crashes, HELP

I recently got tf2, but I haven’t been able to play for more than five minutes at a time due to insanely horrible lagging. The sound files in the background loops, and the game lags in short 0.5 - 3 second intervals at seemingly random times. Eventually the lagging gets so horrible the half the map disappears, the entire game freezes, and I have to get out of it with the task manager. I've tried inputting "-dxlevel 80 +mat_antialias 0" into the launch options, and reducing all the graphics settings to the lowest. There are no other programs running while I play, and my firewall is not the problem (it’s off).

My specs seem just good enough to play the game on regular settings, but here they are anyway: